Abstract/Summary

Summary
• The large-scale loss of Amazonian rainforest under some future climate scenarios
has generally been considered to be driven by increased drying over Amazonia predicted
by some general circulation models (GCMs). However, the importance of
rainfall relative to other drivers has never been formally examined.
• Here, we conducted factorial simulations to ascertain the contributions of four
environmental drivers (precipitation, temperature, humidity and CO2) to simulated
changes in Amazonian vegetation carbon (Cveg), in three dynamic global vegetation
models (DGVMs) forced with climate data based on HadCM3 for four SRES
scenarios.
• Increased temperature was found to be more important than precipitation reduction
in causing losses of Amazonian Cveg in two DGVMs (Hyland and TRIFFID), and
as important as precipitation reduction in a third DGVM (LPJ). Increases in plant
respiration, direct declines in photosynthesis and increases in vapour pressure deficit
(VPD) all contributed to reduce Cveg under high temperature, but the contribution
of each mechanism varied greatly across models. Rising CO2 mitigated much
of the climate-driven biomass losses in the models.
• Additional work is required to constrain model behaviour with experimental data
under conditions of high temperature and drought. Current models may be overly
sensitive to long-term elevated temperatures as they do not account for physiological
acclimation.